Method for the continuous production of isocyanates

ABSTRACT

The invention relates to a process for preparing polyisocyanates by reacting organic amines with phosgene, wherein the reaction is carried out in at least three stages, with the first stage being carried out in a mixing apparatus, the second stage in at least one residence apparatus and the third stage in at least one separation apparatus and the pressure in each successive stage being lower than that in the previous stage.

The present invention relates to a process for preparing aromatic oraliphatic isocyanates. Among aromatic isocyanates, preference is givento methylenedi(phenyl isocyanate) (MDI) and tolylene diisocyanate (TDI),while in the case of aliphatic isocyanates, preference is given tohexamethylene diisocyanats (HDI) and isophorone diisocyanate (IPDI) andothers.

It is an object of the present invention to develop a two-stage ormultistage process which gives isocyanates in very high chemical yieldsand with high space-time yields at a low phosgene holdup.

In the process of the present invention, the reaction between organicamine and phosgene is carried out in two or more stages in an inertsolvent, preferably toluene, chlorobenzene, dichlorobenzene or a mixtureof the latter two, and using an excess of phosgene, wherein the pressureis reduced between each of the stages and the first phosgenation stagecomprises a static mixer, preferably a nozzle, the second stagecomprises a residence apparatus and the third stage comprises a(reaction) column. The pressure upstream of the nozzle is preferably3-70 bar, more preferably 15-45 bar. The residence reactor of the secondstage is preferably operated at a pressure of 2.5-35 bar, preferably15-35 bar. Downstream of the nozzle, the pressure is reduced to thepressure of the second stage by means of a regulating valve or someother device suitable for this purpose. It is also possible to use thenatural pressure drop in the nozzle for reducing the pressure. Thereactor of the first stage, usually a static mixer, can also beintegrated into the reactor of the second stage, viz. a residenceapparatus. The reactor of the third phosgenation stage, usually acolumn, preferably a reaction column, is preferably operated at apressure of 2-20 bar, preferably 3.5-16 bar. Downstream of the reactorof the second stage, the pressure is reduced to the pressure of thereactor of the third stage by means of a regulating valve or some otherdevice suitable for this purpose. Once again, a natural pressure dropmay be sufficient to achieve the reduction in pressure. As reactor ofthe third stage, preference is given to using a (reaction) column as isdescribed, for example, in WO 99/54289 (DE 19817691).

The preparation of isocyanates from amines and phosgene is known. Thereaction is, depending on the type of amine, carried out either in thegas phase or in the liquid phase, either batchwise or continuouspreparation of organic isocyanates by reaction of primary organic amineswith phosgene has been described many times and is carried out on alarge industrial scale (cf., for example, Ullmanns Enzyklopädie derTechnischen Chemie, Volume 7 (Polyurethane), 3^(rd) revised edition,Carl Hanser Verlag, Munich-Vienna, p. 76ff (1993).). The aromaticisocyanates TDI (tolylene diisocyanate) and MDI methylenedi(phenylisocyanate) and PMDI (polymethylene-polyphenylene polyisocyanate) andmixtures of the latter two and the aliphatic isocyanates HDI(hexamethylene diisocyanate) and isophorone diisocyanate (IPDI) inparticular are prepared industrially around the world.

Present-day industrial syntheses of the aromatic diisocyanates MDI andTDI and the aliphatic diisocyanates HDI and IPDI are carried outvirtually exclusively in continuous processes. A continuous process forcarrying out the reaction in a plurality of vessels through which thereaction mixture flows continuously is described, for example, in DE844896. In general, the continuous embodiment of the process is carriedout in two stages. In the first stage of the phosgenation, the amine isreacted with phosgene to form the corresponding carbamoyl chloride andhydrogen chloride and the amine hydrochloride. The reaction betweenamine and phosgene is very fast, strongly exothermic and proceeds evenat very low temperatures. To minimize by-product and solids formation,amine and phosgene, each in an organic solvent if desired, thereforehave to be mixed quickly, which is why the first phosgenation stage isgenerally carried out in a mixing apparatus, preferably a nozzle. Thesecond stage of phosgenation comprises both the decomposition of thecarbamoyl chloride to form the desired isocyanate and hydrogen chlorideand the phosgenation of the amine hydrochloride to form the carbamoylchloride. The temperature of the second phosgenation stage is generallyhigher than that of the first.

The reaction of amine and phosgene in the liquid phase is very fast atall industrially customary temperatures and pressures. For this reason,good mixing of the reactants is sought, in order to suppress secondaryreactions. The phosgenation of primary amines in a mixing reactor asfirst stage of the phosgenation has been described many times.

Mixing apparatuses can basically be classified as dynamic mixers, e.g.stirrers and turbines, and static mixers, rotor-stator systems such asKenics mixers, Schaschlik mixers, SMV mixers, and jet mixers such asnozzles or T-mixers (Chem. Ing. Tech. MS 1708/88, Fortschr. Verf.Technik 23, 1985, 373, Ind. Eng. Chem. Res. 26, 1987, 1184).

Known mixing apparatuses include, in particular, nozzles such as annularslit nozzles (DE 1792660), annular hole nozzles (DE 3744001), smooth-jetmixing nozzles (EP 0065727), fan jet nozzles (DE 2950216), angled jetchamber nozzles (DD 300.168), three-stream nozzles (DD 132340),countercurrent mixing chambers (DE-C 1146872), fluid backup nozzles (FR69428), Venturi mixing nozzles (DE-B 1175666). In-line mixers (U.S. Pat.No. 3,321,283), centrifugal mixing pumps or reaction mixing pumps (EP0291819), tubular reactors (U.S. Pat. No. 3,226,410) or microstructuremixers (EP 0928785) are also known. CA 832432 describes the use ofultrasound waves for mixing.

EP 0830894 describes a mixing reactor for the phosgenation of primaryamines, in which the inlet for one substance is located in the axis ofthe mixing chamber and the inlet for the other substance (or substances)is configured as a multiplicity of nozzles arranged rotationallysymmetrically around the axis of the mixing chamber, with each of thesenozzles having a pin which is movable in the direction of the nozzleaxis and can free the nozzle of adhering solids.

DD 132340 describes a process for the phosgenation of amines to formmonoisocyanates, diisocyanates and polyisocyanates undersuperatmospheric pressure and at elevated temperature in the presence ofa uniform solvent, in which an amine/monochlorobenzene mixture and aphosgene/monochlorobenzene mixture are fed in parallel as a plurality ofsubstreams into a reactor, with part of the phosgene/monochlorobenzenemixture being introduced centrally and the amine/monochlorobenzenemixture being introduced around this central stream and theamine/monochlorobenzene mixture being in turn enclosed by aphosgene/monochlorobenzene mixture. The polyamine/monochlorobenzenemixture is, for example, fed as an annular stream into the phosgenationreactor at 150° C. Before entry into the reactor, a rotating motion isimparted to the mixture by means of an appropriate twist-inducingdevice. In and around the polyamine/monochlorobenzene mixture, aphosgene/monochlorobenzene mixture which has been heated to 150° C. isfed as reactant into the reactor. The relative velocity of the tworeactants is about 15 m/s.

For the second phosgenation stage, which may simultaneously be used asphase separation vessel, a multiplicity of apparatuses has also becomeestablished. The preparation of isocyanates from the correspondingamines by phosgenation is carried out in stirred vessels (e.g. DE-A1468445), in cascades of stirred vessels (DE-C 844896), in packedreaction columns (e.g. WO 99/54289) or in unpacked columns (e.g.Ullmanns Encyclopädie der technischen Chemie, 4^(th) edition (1977),pages 351 ff). In addition, a circulating mode of operation is alsorealised by use of loop reactors in order to provide a sufficientresidence time for complete conversion at a limited reaction volume (andholdup).

The first stage of the isocyanate synthesis is frequently carried out ata very low temperature and the second stage at significantly highertemperature in a residence apparatus. This mode of operation isfrequently described as cold-hot phosgenation and is described, forexample, in W. Siefken, Liebigs Analen der Chemie 562 (1949), page 96.Firstly, a suspension of the intermediates carbamoyl chloride and aminehydrochloride is prepared at low temperature, in particular at 0° C. orroom temperature, but at most 60° C., and this is then converted intothe isocyanate at higher temperatures, in particular in the range100-200° C., in a residence apparatus. Such two-stage processes aredescribed in the Kunststoffhandbuch, Volume 7 (Polyurethane), 3^(rd)revised edition, Carl Hanser Verlag, Munich-Vienna, p. 76ff (1993), and,for example, in DE 2058032, DE 2153268, DE 2908703, DE 1233854.

DE 949227 describes a cold-hot phosgenation process for the continuouspreparation of isocyanates by reaction of amines with phosgene in theliquid phase in the presence of a solvent, in which a solution or slurryof the amine in an inert solvent is continuously combined with liquidphosgene or a solution of phosgene in an inert solvent in a mixingapparatus with intensive stirring and without external cooling in thecold phosgenation and the reaction mixture obtained in this way is thensubjected to the hot phosgenation. As mixing apparatuses, turbomixersand centrifugal pumps and mixing apparatuses having moving mechanicalparts in general are claimed. The residence time in the mixing apparatusranges from a few seconds to one minute.

DE 949228 describes a cold-hot phosgenation process for the continuouspreparation of monocyclic aromatic diisocyanates (e.g. tolylenediisocyanate), in which a suspension is prepared continuously from theparent amine of the isocyanate and the total amount of the solvent usedin the phosgenation, the suspension is continuously reacted cold withphosgene, the reaction product is subsequently pushed continuouslythrough one or more vertical or slanted tubes known as phosgenationtowers in which the material is, if desired with introduction of gaseousphosgene, heated to the phosgenation temperature and the solution issubsequently freed of dissolved phosgene by stripping with a dry inertgas stream in a column. o-Dichlorobenzene was used as solvent. The coldphosgenation is carried out at 0° C. in a stirred vessel, the reactionmixture is then preheated to 30° C. and finally reacted at 170° C. inthe hot phosgenation in two phosgenation towers connected in series togive the isocyanate. Gaseous phosgene is introduced at the bottom of thesecond phosgenation tower. The hydrogen chloride/phosgene/solventmixture taken off at the top of the two phosgenation towers is partiallycondensed in the respective condenser at the top of the respectivephosgenation tower and is recirculated to the bottom. The incondensiblegas comprising phosgene and hydrogen chloride is passed to aphosgene/hydrogen chloride separation unit or to disposal. The liquidand completely phosgenated reaction product leaving the secondphosgenation tower runs into a bubble cap tray column and is there freedof dissolved phosgene by stripping in countercurrent with a stream ofnitrogen. The vapor taken off at the top is passed to a condenser andcondensed solvent is recirculated to the top of the column. At thebottom of the column, the reaction product in the solvent is taken offand passed to distillation.

DE 952086, too, describes a cold-hot phosgenation. The hot phosgenationis carried out in upright reaction towers filled with Raschig rings orother packing elements. The cold phosgenation is carried out at 0° C.and the hot phosgenation is carried out using a rising temperatureprofile from 120° C. to 160° C.

In DE 958558, the circulated solvent is introduced not at the lower endof the reaction towers but in the cold phosgenation stage. Dilution ofthe reaction product from the cold phosgenation gives the advantage of asolution having a relatively low viscosity instead of a viscoussuspension of carbamoyl chloride and amine hydrochloride.

DE 2058032 also describes a cold-hot phosgenation. The hot phosgenationis carried out in horizontal tube reactors at up to about 200° C., andthe reaction mixture from the cold phosgenation is fully reacted in thehot phosgenation stage with continual mechanical mixing and a slowlyrising temperature profile. The cold phosgenation is carried out at 0°C.

U.S. Pat. No. 2,908,703 describes a two-stage process for preparingaromatic isocyanates, in which the first reaction step is carried out at60-90° C., preferably 70-85° C., using chlorobenzene as solvent, and thesecond reaction step is carried out at a temperature which is highenough to decompose the intermediate to form the isocyanate. A solutionof amine in an organic solvent, preferably chlorobenzene oro-dichlorobenzene, and gaseous phosgene are introduced simultaneouslyinto a stirred and heated reactor so that a saturated phosgene solutionis formed and the phosgene excess is at least 50% over thestoichiometric amount of phosgene for the amine/phosgene reaction. Thereaction mixture is subsequently heated to the decomposition temperatureof the carbamoyl chloride and amine hydrochloride. The isocyanate formedis finally separated off by fractional distillation or other methods. Asan alternativt to this batch procedure, the process described can alsobe carried out continuously by carrying out the first reaction step in afirst reactor and the second step in a second reactor. The reaction isgenerally carried out at atmospheric pressure. The concentration of theamine in the organic solvent is 2-20% by weight, preferably 5-10% byweight. Higher concentrations lead to formation of by-products, inparticular ureas and polyureas.

In U.S. Pat. No. 3,381,025, the phosgenation of an organic primary amineto form the corresponding isocyanate is carried out in two stages at<60° C. in the first stage and 100-190° C. in the second stage. Amixture of the inert solvent, excess phosgene and the hydrogen chlorideformed is taken off from the second reaction stage and the hydrogenchloride is separated off from this mixture by cooling the mixture to−20° C. The cold liquid mixture of phosgene and solvent obtained isrecirculated to the first reaction stage.

DE 2153268 describes a process for a continuous cold phosgenation oforganic primary amines by reaction with a solution of phosgene in aninert solvent in a multistage, not self-priming centrifugal pump. Thecentrifugal pump at the same time conveys the resulting reaction mixtureto the subsequent hot phosgenation stage. The phosgene solution entersthe pump at from −105° C. to +25° C. and the amine solution enters it atfrom 50 to 100° C. The reaction mixture leaves the pump at from 50 to110° C. The concentration of the amine solution is from 5 to 40% byweight, and that of the phosgene solution is 20-65% by weight. Theamount of phosgene is at least 1 mol, preferably 1.5-3 mol, per mol ofamine group.

A disadvantage of the two-stage procedure with a low temperature in thefirst stage and a high temperature in the second stage (cold-hotphosgenation) is the low reaction rates and thus low space-time yieldsowing to the low temperatures in the first phosgenation stage. The lowtemperatures (high solubility of phosgene) and the long reaction times(large reactors) additionally imply a high phosgene holdup, which isundesirable from a safety point of view. Low temperatures are alsoproblematical because of the massive precipitation of the carbamoylwhich is formed as an intermediate and decomposes rapidly at elevatedtemperatures. This brings with it the risk of blockages and theformation of caked material. Furthermore, cooling of the reactants andlater heating of the reaction mixture is energetically disadvantageous.To achieve economical space-time yields, operation at elevatedtemperature in all stages is necessary in industrial processes forpreparing organic isocyanates by phosgenation of primary organic amines.However, the solubility of the phosgene in the reaction mixture andtherefore the phosgene excess available for the reaction decreases athigh temperatures, since the reaction generally takes place in theliquid phase. A high excess of phosgene is nevertheless necessary toachieve high yields of isocyanate. EP 0716079 describes the influence ofpressure and temperature on the reaction and the phosgene excess. Thelowering of the phosgene excess at elevated temperatures is generallycountered by means of an increased reaction pressure. DE-A 1768439describes a process for the continuous preparation of organicisocyanates which employs a combination of high temperature above 180°C. and a high pressure of from 20 to 150 atm together with a highphosgene concentration in the reaction zone. The amount of amineintroduced is from 2.5 to 5.5 times the stoichiometric amount. As aresult of the extremely high pressure and the very high temperature,acceptable space-time yields can be achieved. The residence time of thereactants in the reaction zone is 5-60 s. The preferred solvent ischlorobenzene. A disadvantage of the process is the reduced yield andquality caused by the increased formation of by-products, in particularureas, as a result of the high temperature. In addition, industrialpressure apparatuses are very expensive and are problematical because ofthe high toxicity of phosgene. Carrying out the reaction of amine withphosgene at high pressure also has the disadvantage that, in accordancewith Henry's law, not only the phosgene concentration but also thehydrogen chloride concentration in the liquid phase is increased. Theincreased formation of amine hydrochlorides is, however, undesirablesince their phosgenation is, according to generally accepted opinion,very slow and therefore represents the rate-determining step of theoverall reaction. This leads to very long residence times and largephosgene holdups.

EP 0065727 describes a process using a nozzle and tube reactor. Aprocess for the continuous preparation of organic monoisocyanates andpolyisocyanates in a single-stage reaction by continuous combination ofsolutions of primary monoamines or polyamines in inert organic solventswith excess amounts of phosgene dissolved in an inert organic solvent atpressures of from 10 to 1000 bar, preferably from 25 to 150 bar, andtemperatures of from 120 to 300° C., preferably from 150 to 250° C., ina mixing zone and, if desired, a downstream reaction zone and continuouswork-up, with the phosgene solution, which is used in excess, beingcontinuously placed in a mixing zone and the amine component, which isused in a deficiency, being injected by means of a smooth-jet nozzle, isdescribed. The smooth-jet nozzle which is essential to the process hasan internal diameter of 0.1-30 mm. A differential pressure of at least0.5 bar, preferably 1-200 bar, in particular 3-50 bar, is maintained inthe phosgene solution injected. The molar ratios of phosgene to aminogroups are from 2:1 to 30:1, preferably from 3:1 to 18:1. Theafter-reaction zone can be a tube reactor, a shell-and-tube reactor or,for example, a cascade of stirred vessels. The mean residence time inthe mixing vessel and in the downstream reaction zone is from 5 secondsto 5 minutes. The reaction mixture leaving the after-reaction zone isdepressurized to atmospheric pressure in one or more stages in adepressurization vessel, resulting in a temperature drop of 50-150° C.Gas and liquid phases are separated in the depressurization vessel. Assolvents, preference is given to using chlorobenzene oro-dichlorobenzene.

GB 827376 describes a continuous process for preparing aromaticisocyanates by reacting an amine in free form in a solvent or as a saltwhich readily decomposes to the amine and is suspended in the solventwith a solution of phosgene in an inert solvent at a pressure of greaterthan 3*10⁵ Pa, with the reactants being introduced simultaneously withmixing into the lower end of a vertical tube reactor in which thereaction products rise rapidly to the upper end. The liquid phase iscollected in a container from which it is taken off to isolate theisocyanate. This container can be a phase separation apparatus which isoperated under the same pressure, is connected via an overflow tube tothe liquid outlet and has a throttle valve in the liquid outlet. Theliquid collected in the container is fed into a column which is operatedunder atmospheric or superatmospheric pressure and at elevatedtemperature, so that residual phosgene and hydrogen chloride areseparated off at the top in gaseous form. The excess phosgene iscondensed out (preferably by means of cooling water) from the hydrogenchloride/phosgene mixture separated off in the container, and thehydrogen chloride which has been separated off in this way isdepressurized and discharged. The reactants are fed by means of onejoint pump or two separate pumps into the tube reactor or else mixed ina Venturi mixing nozzle, preferably one having separate inlets for thetwo reactants, and from there introduced into the tube reactor. Thetemperature in the tube reactor is given as 80-200° C., the pressure isgreater than 3*10⁵ Pa, at most the vapor pressure of the reactionmixture and preferably 15-20*10⁵ Pa.

U.S. Pat. No. 3,226,410 describes a continuous process for preparingaromatic isocyanates by mixing a stream of an aromatic amine into aphosgene stream in a tube reactor at Reynolds numbers of greater than2100 (preferably 5000-2000000) and temperatures of 60-90° C., preferably80-85° C. The amount of phosgene is at least 1 mol, preferably from 6 to12 mol, per mol of amine. The reaction solution is then, if desiredafter preheating, fed into a second reactor, in particular a vessel or acolumn, which is at from 110 to 135° C., preferably from 110 to 120° C.The amine concentration is from 2 to 25% by weight, preferably from 5 to10% by weight, and the phosgene concentration is from 10 to 100% byweight, preferably from 10 to 60% by weight. The pressure at which thephosgene stream is introduced into the tube reactor is 50-170 psig; thepressure of the amine stream has to be greater in order to preventbackmixing. The liquid phase comprising isocyanate, solvent, relativelysmall amounts of by-products, hydrogen chloride and phosgene dissolvedin a solvent is taken off from the second reactor separately from thegas phase comprising hydrogen chloride, solvent, phosgene and traces ofthe isocyanate. Solvents used are chlorinated hydrocarbons which areinert and have a boiling point lower than that of the isocyanate.Particular preference is given to chlorobenzene.

The second reactor, which has a pressure of 45 psig or higher, isfollowed by a residence vessel and a buffer vessel from which the liquidphase is conveyed under level control to a column for removing excessphosgene. Phosgene, hydrogen chloride and solvent are taken off at thetop and recirculated to the phosgene container. The bottom productcomprising isocyanate and solvent is conveyed to a distillation,preferably a single-stage distillation, to separate off the solvent. Thesolvent which has been separated from the isocyanate is used forabsorption of the remaining phosgene from the hydrogen chloride stream.The phosgene taken off in the second reactor and in the buffer vessel iscondensed in two stages and recirculated to the phosgene container. Theuncondensed phosgene/hydrogen chloride mixture is conveyed to a phosgeneabsorber into which solvent recovered in the solvent separation is fed.The gas which has not been absorbed, mainly hydrogen chloride, issubsequently reacted with water in an absorber to form aqueoushydrochloric acid.

The tube reactor should be constructed as a plug flow reactor withoutdeflection plates, pockets or other internals which can produce deadzones so that settling of solids is prevented. The high Reynolds numbersand the design of the reactor as straight tubes are intended to lead tothe liquid continually keeping the walls free of caked material.

DE 952086 describes a process for preparing isocyanates from primaryamines or their salts and phosgene, in which the reactants are, in thehot phosgenation in the presence of a solvent or diluent, passedcontinuously from the bottom upward through a vertical or slanted,heated tube. After the reactants have passed through this first reactiontube, they can, if appropriate, pass through a second, likewise verticalreaction tower with addition of further phosgene to complete thereaction. The advantage of vertical reaction towers which are filledwith Raschig rings or other packing elements to prevent rapid degassingis that the rate of isocyanate formation is increased as a result of theincreased phosgene concentration at the entry point for the reactantslocated at the bottom due to the hydrostatic pressure of the column ofliquid. o-Dichlorobenzene is named as solvent. The process is atwo-stage process with a cold phosgenation as first stage and a hotphosgenation as second stage. The cold phosgenation is carried out at 0°C. and the hot phosgenation is carried out with a rising temperatureprofile from 120° C. to 160° C.

Disadvantages of this process are the fundamental weak points ofcold-hot phosgenation and also the low achievable pressure. Considerablyhigher pressures and thus phosgene concentrations in the liquid phasecan be obtained by means of pressure vessels and regulating valves. Afurther disadvantage is the packing elements used in the tube reactors,since the solid intermediates carbamoyl chloride and amine hydrochlorideformed and precipitated in the cold phosgenation can easily lead toblockages and thus to low availability of the plant.

DE 2058032 describes a process for preparing isocyanates from amines andphosgene in the presence of an inert solvent, in which the reactionmixture is treated at 0° C. in the cold phosgenation and is subsequentlyreacted fully at up to about 200° C. in the hot phosgenation, with thereaction mixture from the cold phosgenation being conveyed in the hotphosgenation stage through a horizontal reaction zone with continualmechanical mixing and a slowly rising temperature profile. As heatablereaction vessel for the hot phosgenation, a horizontal tube throughwhich a shaft provided with tube devices extends in the longitudinaldirection and which has separately heatable sections of wall heating isclaimed. A natural convection vaporizer (thermosyphon) can be located atthe end of the tube reactor. An arrangement of three reaction tubesconnected to one another in the shape of a U (system of communicatingtubes) is also described. The cold phosgenation is carried out in thefirst, vertical tube, the hot phosgenation is carried out in thehorizontal tube and the degassing, i.e. the removal of hydrogen chloridephosgene from the reaction mixture, is carried out in the secondvertical tube. Transport pumps and regulating valves can be omitted insuch a system, since the product flow is established according to theinflow rate.

DE 2747524 describes a continuous process for preparing aromaticisocyanates in which heat is introduced into the reactor at a rate sothat added phosgene does not lead to cooling and thus to caking of thecarbamoyl chlorid intermediate on the reactor wall. A plug flow reactorcomprising two coaxial tubes into which the two reactants amine andphosgene in an inert organic solvent are introduced in countercurrent,each isolated from the other, and are mixed at the end of the internaltube is described. Backmixing into the feed zone is said to be preventedso as to minimize by-product formation. Heating is by means of a steamjacket in order to prevent blocking of the mixing zone by the carbamoylchloride intermediate. Temperatures of 90-140° C. are said to benecesssary; in general, temperatures of 90-200° C. are indicated. Theinitial temperature is, howeveer, 60-90° C. Practical consideerationsdetermine the upper limit to the temperature. 2 atmospheres gauge isindicated as a convenient pressure. The amine concentration in the inertsolvent is given as from 2 to 20%, preferably from 5 to 10%.Dichlorobenzene is preferred as inert solvent.

A tube reactor is also the preferred apparatus in the process describedin WO 96/16028 for the preparation of isocyanates using isocyanate assolvent. WO 96/16028 describes a continuous, single-stage process inwhich the primary amine, optionally dissolved in an inert, organicsolvent, is reacted with phosgene dissolved in the isocyanate in aconcentration of 10-60% by weight, based on the isocyanate/phosgenesolution, at 60-180° C. and pressures of 1-30 bar to form thecorresponding isocyanate, with the molar ratio of phosgene to amine usedbeing from 4:1 to 1:1 and the isocyanate used as solvent being free ofsolids and having a hydrolyzable chlorine content of less than 2%.

DE 19817691 describes a two-stage process for preparing mixtures ofdiphenylmethane diisocyanates (MDI) and polyphenylene-polymethylenepolyisocyanates (PMDI) having a reduced content of chlorinatedby-products and a reduced iodine color number by two-stage reaction ofthe corresponding mixtures of diphenylmethanediamines (MDA) andpolyphenylene-polymethylene-diamines (PMDA) with phosgene in thepresence of at least one organic solvent at elevated temperature,removal of the excess phosgene and solvent after the phosgenation iscomplete and thermal treatment of the reaction product with the molarratio of phosgene to hydrogen chloride in the residence apparatus of thesecond stage of the phosgenation being 10-30:1 in the liquid phase andat the same time 1-10:1 in the gas phase. The carbamoyl chlorides andamine hydrochlorides formed in the first stage of the phosgenation, viz.static mixer, pass through a residence apparatus in the second stage ofthe phosgenation in which the amine hydrochlorides are phosgenated tothe corresponding carbamoyl chorides and the carbamoyl chlorides aredissociated into the corresponding isocyanates and hydrogen chloride.The temperature of the first stage is usually from 40 to 150° C.,preferably from 60 to 130° C., particularly preferably from 90 to 120°C. Static mixers employed for the first stage are, in particular,nozzles. As residence apparatus for the second stage, use is made ofstirred apparatuses, cascades of stirred vessels and particularlypreferably a column, in particular a reaction column usually having <10theoretical plates. It is particularly advantageous to operate thiscolumn in countercurrent. The temperature at the bottom of the column ispreferably from 80 to 120° C., particularly preferably from 90 to 110°C. The pressure at the top of the column is preferably from 1.0 to 4.7atm (gauge), particularly preferably from 2.0 to 3.7 atm (gauge).

A disadvantage of this process is that the amine hydrochloridephosgenation and the carbamoyl chloride decomposition are carried out inone and the same reactor, which leads to prolonged residence times andhigher phosgene holdups.

U.S. Pat. No. 3,544,611 describes a process for preparing organicisocyanates at a high pressure of from 10 to 50 bar. It was surprisinglyfound that carrying out the reaction at relatively high pressures of atleast 10 atm gauge leads to higher yields of isocyanate. Furthermore,higher pressures aid the hydrogen chloride/phosgene separation. Thefirst reaction step of the isocyanate preparation, viz. the reactionbetween amine and phosgene to form the carbamoyl chloride intermediate,is carried out in a loop reactor (mixing circuit). The second reactionstep, viz. the decomposition of the carbamoyl chloride to form theisocyanate, occurs in a reaction column located downstream of the mixingcircuit. A hydrogen chloride-phosgene mixture is obtained at the top ofthis column. Phosgene is condensed in two stages from this mixture. Thephosgene which has been condensed out is recirculated to the top of thecolumn. At a liquid offtake in the enrichment section of the column,phosgene is taken off and recirculated to the reaction (the mixingcircuit).

The remaining phosgene is separated off from the reaction mixture takenoff at the bottom of the reaction column in a further column. In thelatter, phosgene is taken off at the top, condensed in two stages in amanner analogous to the first column and recirculated to the reaction inthe mixing circuit. The reaction to give the isocyanate is finished inthe reaction column.

DE 3736988 describes a continuous process for preparing organicmonoisocyanates or polyisocyanates in a single-stage reaction byreacting the amine dissolved in an organic solvent with phosgenedissolved in an organic solvent in a reaction column at below 150° C.The reaction mixture is allowed to pass continuously through thereaction column from the bottom upward. The reaction column has at least10 chambers separated from one another by perforated plates. Theconcentration of the amine in the inert solvent is 5-40% by weight,preferably 7-20% by weight. Preferred solvents are chlorobenzene ordichlorobenzene or mixtures thereof. Phosgene is used as a 30-65%strength by weight, preferably 40-65% strength by weight, solution inthe inert solvent. The equivalence ratio of amine to phosgene is from1:1.5 to 1:7, preferably from 1:2 to 1:5. The temperature at the top ofthe column is preferably 70-130° C., particularly preferably 90-125° C.,and not more than 150° C. The mean residence time in the reaction columnis not more than 120 minutes, preferably not more than 60 minutes. Thepressure in the column is 1.2-3 bar abs, preferably 1.5-2.5 bar abs.

DE 3744001 likewise proposes a perforated plate column having >10perforated plates, preferably 20-50 perforated plates, as residenceapparatus through which the reaction mixture flows from the bottomupward at a liquid velocity of 0.05-0.4 m/s, preferably 0.1-0.4 m/s, anda gas velocity of 2-20 m/s, preferably 3.5-10 m/s, and a residence timeof not more than 120 minutes, preferably not more than 60 minutes. Thehorizontally installed perforated plates form 10-50 chambers. Thetemperature at the top of the reaction column is less than 150° C.,preferably 70-130° C., particularly preferably 90-125° C. The pressureat the top of the column is 1.2-3 bar (abs.), preferably 1.5-2.5 bar(abs.). A mixing nozzle is claimed for the first phosgenation stage.

EP 0291819 describes a two-stage process for preparing isocyanates bycold-hot phosgenation, in which a mixer having a rotary disk is used forthe cold phosgenation and phosgenation towers are preferably used forthe hot phosgenation. The phosgenation towers are operated atatmospheric pressure or a slightly superatmospheric pressure of up to1.5 atm gauge. It is particularly advantageous to allow the reactionmixture from the cold phosgenation leaving the mixing apparatus to entera heatable tower continuously at the top or the bottom and complete thereaction by introduction of heat. To set a particular temperatureprofile, a plurality of towers can be connected in series or acombination of towers and vessels can be used.

In DE 2112181 (U.S. Pat. No. 3,829,458), organic isocyanates areprepared continuously from primary organic amines and Phosgene in aninert organic solvent in one or more reaction vessels which containpacking and through which the reactants flow in cocurrent in atransition stream. The transition stream consists of anamine-containing, liquid organic phase and a phosgene-containing gasphase. The reaction takes place at reaction temperatures in the rangefrom 50 to 220° C. In the case of incomplete reaction, the reactionmixture is circulated a number of times through the packed column. Adisadvantage of this process is the high susceptibility of the packedcolumn to blockages caused by solids such as carbamoyl chloride, aminehydrochloride, ureas, etc., being deposited on the packing elements.Furthermore, packed columns have a high pressure drop, which results inhigh temperatures at the bottom and thus high thermal stress on thereaction mixture and the isocyanate formed, leading to increasedby-product formation and a reduced yield.

In many processes, the reaction of phosgene and amine is carried out ina loop reactor or circulating reactor in which not only the feed streamsof amine and phosgene, if desired in a solvent, but also at least partof the reaction mixture are recycled. This dilution by recirculation ofthe reaction mixture formed serves mainly to control the temperature andto achieve better removal of heat so as to set low temperatures. Thereaction between amines and phosgene is strongly exothermic. In thecases of unfavorable reaction conditions and configuration of theapparatus, higher temperatures cause increased by-product formationwhich, for example in the case of tolylene diisocyanate (TD), lead to adecrease in yield and formation of tar. Main by-products formed areureas.

DE 2624285 (BASF) describes a mixing circuit process for the continuouspreparation of organic isocyanates from organic amines and phosgene inthe presence of organic solvents, in which the phosgene is mixed intothe circulated reaction solution and the reaction mixture obtained andthe amines or amine solution are fed into the mixing and reaction zonein such a way that an energy dissipation density of from 5 to 1000 kJper m³ of recirculated reaction mixture plus introduced amine solutionis produced. The reaction is carried out at from 90 to 220° C.,preferably from 120 to 180° C., and in a pressure range from 1 to 10bar, preferably from 1 to 3 bar. The residence times are from 10 to 180minutes. The molar ratio of amine to phosgene is such that from 1 to 10mol, preferably from 1.3 to 4 mol, of phosgene per amino group arepresent in the reaction mixture. The yields are from 88 to 98% byweight, based on the amine used.

The mixing circuit process described in DE 2624285 is developed furtherin EP 0150435. In the process for the continuous preparation of organicisocyanates by reaction of organic amines with phosgene in the presenceof organic solvents, with hydrogen chloride being separated off and thereaction mixture being partly circulated, the hydrogen chloride contentof the reaction mixture recirculated to the addition of amine after thehydrogen chloride has been separated off is, prior to the addition ofamine, equal to or less than 0.5% by weight, preferably from 0.01 to0.4% by weight, based on the total weight of the reaction mixture, andthe molar ratio of phosgene to amino groups of the organic amines is12-200:1. The reaction is carried out at 100-220° C., preferably120-180° C., and in a pressure range of 5-100 bar, preferably 15-50 bar.

DE 3403204 describes a process for the continuous preparation of organicisocyanates, preferably polyisocyanates, by reaction of organic amines,preferably polyamines, with phosgene in the presence of organic solventsunder superatmospheric pressure, e.g. from 5 to 100 bar, and elevatedtemperatures, e.g. from 100 to 220° C., with the reaction mixture beingpartly circulated, preferably by the natural convection principle, andthe hydrogen chloride content of the reaction mixture prior to theaddition of amine being less than 0.5% by weight, based on the totalweight of the reaction mixture, and the molar ratio of phosgene to aminogroups of the organic amines being 12-200:1.

DE 3212510 describes a process for the continuous preparation of organicisocyanates. The primary organic amine is brought into contact in avirtually dispersed state with an excess of phosgene at a gauge pressureof 10 kg/cm², approximately 10 bar, and a temperature of from 60 to 100°C., resulting in formation of a corresponding organic carbamoyl chloridefrom the organic amine and hydrochloride formed as intermediate.Hydrogen chloride is formed as by-product. In this first stage of thereaction, from 30 to 70% of the carbamoyl chloride is converted intoisocyanate. The reaction mixture is maintained at a gauge pressure of 10kg/cm² and a temperature of from 120 to 160° C., so that thehydrochloride is converted into carbamoyl chloride and the conversion ofthe carbamoyl chloride into isocyanate is complete. The reaction takesplace in a circulation reactor (circulation line) or in a tank-shapedreaction vessel. In the first case, the phosgene is allowed to circulatetogether with the solvent in a tubular circulation line and the amine ismixed in in this (mixing circuit). The residence time in the first stageis 30-120 minutes and that in the second stage is 10-120 minutes.ortho-Dichlorobenzene is chosen as solvent.

GB 763535 and DE 1811609 likewise describe loop reactors or circulationreactors (mixing circuits as reaction system). The organic isocyanate isprepared by reacting an amine with phosgene in a single-stage continuousreaction with circulation of isocyanate, solvent and unreacted phosgene.The pressure described as sufficient in the process described in GB763535 is 5-20 pounds per square inch, the reaction temperature is90-180° C., the TDA concentration in the solvent is 5-30%, thestoichiometric excess of phosgene is at least 25%, preferably 70-110%,and the solvents used are chlorinated aromatic hydrocarbons, preferablyo-dichlorobenzene. In DE 1811609, the organic amine, if desired inortho-dichlorobenzene or another solvent, and an excess of phosgene aremixed under high shear stress into the circulating reaction mixture, bywhich means conditions which advantageously deviate from GB 763535 canbe set owing to the mixing. The reaction pressure is preferably at least1.8-14*10⁵ Pa, preferably 4.2*10⁵ Pa or 3.5*10⁵ Pa. The reactiontemperature is stated to be preferably 102-130° C. and, in the case oftoluenediamine, preferably 90-120° C. The excess of phosgene is 50-200%,preferably 70%.

DE 1037444 (U.S. Pat. No. 2,822,373) describes a continuous process forpreparing organic isocyanates, in which a solution of the organic aminein an inert solvent is reacted with a solution of phosgene in an inertsolvent at from 90 to 180° C. in a reaction zone in whichsuperatmospheric pressure and turbulent flow prevail. The reactionsolution is then depressurized into a zone at a lower pressure, usuallyatmospheric pressure, and hydrogen chloride and phosgene are taken offas a gaseous mixture. The isocyanate is separated from the solvent bydistillation. In the process described, the amine solution is introducedinto a pumped circulation line, the circulating mixture is heateddownstream of the reaction zone by means of a heat exchanger and is thendepressurized via a throttle valve into a tank-shaped residence vessel.From this, the reaction mixture is either taken out for the pumpedcircuit or partly discharged for removal of solvent and recovery of theisocyanate. The solvent is condensed from the gaseous mixture ofhydrogen chloride, excess phosgene and solvent taken off from thereservoir in a condenser and is returned to the reservoir. Preferredpressures in the pumped circulation line are given as 5-20 pounds persquare inch. The amine concentration in the solvent is 5-30% by weight,and the amount of phosgene is at least 1.25 mol per amino group of theamine. The preferred solvent is stated to be ortho-dichlorobenzene.

U.S. Pat. No. 3,574,695 describes an improved continuous process forpreparing organic isocyanates. The residence times can be shortened bytreating the product from the first reaction zone in the second reactionzone with at least 0.75 mol of phosgene per equivalent of organic amineadded in the first reaction zone. The residence time in this secondreaction zone is 5-45 minutes at a temperature of at least 130° C. Agaseous mixture of hydrogen chloride and phosgene and a liquid solutionof the organic isocyanate in the solvent are continuously taken off fromthe second reaction zone.

GB 1034285 describes a continuous process for preparing organicisocyanates by reaction of phosgene with a primary polyamine in thepresence of an inert organic solvent, with the reactants being fedseparately into a tube reactor and brought into contact there and amixture of the same solvent, the reaction mixture and phosgene beingrecirculated through this tube reactor. As reactor, it is possible touse an assembly of two cylindrical vessels between which the reactionmixture is circulated or an annular tube reactor. The reaction mixturecan be stirred by means of stirrers. The temperature in the tube reactoris 8-50° C. The pressure is atmospheric pressure or slightly above this.The concentration of the introduced primary polyamine in the solvent is2-20% by weight. The amount of phosgene added to the stream circulatedby pumping is from 5 to 20 mol of phosgene per mol of amino groups inthe polyamine solution added. As inert organic solvent, use is made ofchlorobenzene or ortho-dichlorobenzene.

GB 1212249 describes a process for preparing isocyanates in which anexcess of phosgene is reacted with a forward-directed, mixed film of theamine in an inert solvent in the first stage. A cylindrical tube isregarded as suitable for producing this film.

JP 57-048954 describes a process for preparing organic isocyanates inwhich the solution of a primary amine is introduced just upstream of thefeed point of a static mixer which is located in a circulation reactor.A solution of phosgene in an organic isocyanate circulates in thecirculation reactor.

Phosgenation in a loop reactor is also described in JP 60-10774 in whichan isocyanate-containing reaction mixture is circulated by pumping.However, high yields are only achieved at amine concentrations of 5-10%.

The low temperatures in the first stage and the high temperatures in thesecond stage of loop reactor or mixing circuit processes areenergetically disadvantageous. Since the reaction between an organicamine and phosgene is strongly exothermic, intensive cooling has to beemployed in the first step in order to maintain the desired reactiontemperature. The second reaction, viz. the decomposition of thecarbamoyl chloride to form the isocyanate, is significantly endothermic,so that the reaction mixture has to be heated again in the second stage.

Moreover, the significantly lower chemical yield compared to processescarried out in a single pass is particularly disadvantageous, sinceisocyanate already formed reacts with amine to form ureas in the mixingcircuit due to backmixing. To suppress this secondary reaction, a lowmaximum steady-state isocyanate concentration is frequently permitted,but this in turn results in low space-time yields.

In the process described in EP 0716079 for the continuous preparation oforganic isocyanates, the reaction mixture is circulated at 60-100° C. ina bubble column. The process described is carried out at slightlysubatmospheric or slightly superatmospheric pressure, in general at0.5-5 bar, preferably 1-3 bar.

EP 0570799 describes a process in which the reaction between amine andphosgene to form isocyanate is carried out in the gas phase. Gas-phasephosgenation is known for the preparation of aliphatic diisocyanates (EP0289840,), aromatic diisocyanates (EP 0570799), cyclic diisocyanates (EP1078918) and of triisocyanates (EP 0749958). EP 0749958, EP 0676392 andEP 0289840 describe processes for preparing aliphatic diisocyanates andtriisocyanates by gas-phase phosgenation in which the reactants aremixed between nozzle and tube on entering the tube reactor describedthrough nozzles or a combination of nozzles and an annular gap. AReynolds number of RE>4700 in the tube is indicated as an essentialcriterion for mixing. A jet mixer is proposed in EP 0570799 for thepreparation of aromatic diisocyanates by gas-phase phosgenation.

DE 1192641 describes a process for preparing isocyanates by reaction ofprimary aromatic or araliphatic amines with phosgene in the presence ofsolvents and subsequent heating of the reaction mixture, with theisocyanate which is to be prepared in the reaction being used assolvent.

DE 100 27 779 claims a process for preparing isocyanates by reactingamines with phosgene, in which the isocyanate is used as solvent and thereaction is carried out in a reaction column and the condensed phase atthe bottom of the reaction column is recirculated in its entirety or inpart to the enrichment section of the reaction column. The number oftheoretical plates in the reaction column is 5-60. The temperature isfrom −20° C. to 300° C. and the absolute pressure is 0.2-60 bar.

U.S. Pat. No. 2,683,160 describes a process for preparing aromaticrisocyanates in which gaseous phosgene and a solution of an aromaticamine in a chlorinated aromatic hydrocarbon as solvent are introducedsimultaneously into a solution of the desired isocyanate in theabovementioned solvent. The solution of the desired isocyanate in thesolvent is maintained at the boiling point of the solvent, i.e. at 130°C.-300° C., with the solution boiling under reflux. The by-producthydrogen chloride and excess, unreacted phosgene are taken offcontinuously through the reflux condenser. The excess of phosgene is atleast 50%, preferably from 80 to 100%, of the stoichiometric amount. Theamine concentration in the solvent is from 5 to 30%, preferably from 8to 12%. The reaction is carried out in a single apparatus which isstirred well and heated sufficiently.

DE 2252068 describes a process for preparing organic isocyanates fromamine and phosgene, in which the amine which has been preheated to atemperature below its decomposition temperature at superatmosphericpressure is firstly reacted with preheated phosgene in the presence ofan excess of an organic isocyanate as solvent at temperatures andpressures such that the reaction proceeds in a homogeneous, liquid phaseand the organic carbamoyl chloride formed as intermediate issubsequently thermally dissociated at a lower pressure in a secondstage. In a preferred embodiment, the first reaction stage is carriedout adiabatically. The reaction components are fed in at temperatures inthe range 120-180° C. The temperature of the reaction mixture at theoutlet is maintained at 180-250° C. and the pressure is maintained at100-300 atm. The residence time of the components in the first reactionzone should be 5-150 seconds. The second reaction stage is carried outisothermally. The feed temperature is 120-250° C. and the pressure is3-30 atm. The residence time is 3-30 minutes. The isocyanate taken offfrom the second stage is cooled to 50-80° C. prior to recirculation.

U.S. Pat. No. 3,801,518 describes a process for preparing tolylenediisocyanate having an increased acidity of 0.03-0.3% by weight. This isachieved by phosgenation of toluenediamine and subsequent residence ofthe reaction product in a phosgene atmosphere at 100-200° C. for aperiod of at least 0.08 hour, preferably from 0.08 hour to 2 hours.

U.S. Pat. No. 3,912,600 describes the reduction in the acidity and thecontent of hydrolyzable chlorine in a polymethylene-polyphenylenepolyisocyanate (PMDI) by treatment of this in an inert, organic solventat a pressure of 20-60 psia and a temperature of 150-230° C., withremoval of volatile compounds, known as low boilers.

GB 1196008 describes a continuous process for preparing aromaticmonoisocyanates or diisocyanates by phosgenation of the correspondingamines in an organic solvent at 120-200° C. in two reaction vesselscoupled to one another, with the excess of phosgene over the calculatedstoichiometric amount being 5-20%.

It is an object of the present invention to develop a two-stage ormultistage process which gives isocyanates in very high chemical yieldsand with high space-time yields at a low phosgene holdup.

It has surprisingly been found that, contrary to generally prevailingopinion, the second reaction, viz. the phosgenation of the aminehydrochloride, proceeds at a high reaction rate at high phosgeneconcentrations and elevated temperatures. High pressures are thereforeadvantageous for this reaction, since high pressures imply high phosgeneconcentrations in the liquid phase. Furthermore, elevated temperaturesare advantageous for achieving high space-time yields. I. I.Konstantinov, A. I. Kormucheshkina, Zhurnal Prikladnoi Khimii, 49 (3),pp. 596-599, 1976) state that the phosgenation of the aminehydrochloride is very slow and is the rate-determining step of theoverall reaction cycle to the isocyanate. Konstantinov et al. presentkinetic measurements and quantify the reaction rates. According to them,the reaction rate for the phosgenation of the hydrochloride isconsiderably lower than that for the free amine. As described in GB1212249, the formation of amine hydrochloride also leads to a loss ofisocyanate yield due to urea formation. Since the solubility of aminehydrochlorides in the corresponding reaction mixtures and also in mostcommercially available solvents is very low, hydrochloride formationalso drastically increases the problem of solids formation.

DE 3323882 describes a continuous process for the hot phosgenation ofamine hydrochloride or mixtures thereof with carbamoyl chloridesuspended in solvents by means of excess phosgene at from 80° C. to 200°C., preferably from 100° C. to 180° C. In this process, the solids areretained in the reactor by means of a suitable separation device and theisocyanate which is formed during the reaction and is present as asolution in the solvent is continuously taken off from the reactor. Thesolids are preferably separated off by means of a filter. Disadvantagesof this process for phosgenation of hydrochlorides are the complicatedsolids handling, the risk of blockage of pipes and, in particular, ofregulating valves and flowmeters and also the long residence time whichrequires large apparatuses and results in a high phosgene holdup andalso the severe reaction conditions and the relatively low yields.

DE 2404773 describes a process for preparing monoisocyanates,diisocyanates and/or polyisocyanates from organic primary amines andphosgene, in which the primary amines are mixed with at least 3 mol ofphosgene per amino group in the absence of a solvent and the reactionmixture is simultaneously broken up to a mean particle size of 1-100 μmand the resulting suspension of carbamoyl chloride and aminehydrochloride in phosgene is converted into the correspondingisocyanates at from 100 to 180° C., preferably from 120 to 160° C., andpressures of from 14 to 55 bar, preferably from 21 to 41 bar. Theprocess is a two-stage process in which the starting materials primaryamine and phosgene are mixed in the first step at from −30° C. to 60°C., preferably 0-50° C., at atmospheric pressure or preferablysuperatmospheric pressure, in particular at from 14 to 55 bar, and atthe same time the particles are comminuted to a mean particle size offrom 1 to 100 μm, preferably from 1 to 50 μm. The amine is added as aliquid, melt or possibly as a powder to the phosgene. Various mixing andcomminution devices are described. The second stage comprises thereaction of amine hydrochloride with phosgene to form carbamoyl chlorideand its decomposition into isocyanate and hydrogen chloride in apressure vessel at 100-180° C., preferably from 120 to 160° C., andpressures of 14-55 bar, preferably from 21 to 41 bar. This process istechnically very complicated and not economical.

DE-A 156844 likewise describes a phosgenation of an amine hydrochloridesuspension which is carried out at elevated temperature in a multistagecascade of stirred vessels. A particular disadvantage of a cascade ofstirred vessels is the high phosgene holdup.

It has now surprisingly been found that the phosgenation of the aminehydrochloride is a fast reaction at high phosgene concentrations andelevated temperatures.

Furthermore, it has surprsingly been found that amine hydrochloride andcarbamoyl chloride formed in situ display a high level ofsupersaturation in the reaction mixture when a static mixer, preferablya nozzle, is used as reactor for the first stage. Even when aminehydrochloride and/or carbamoyl chloride precipitate, the process claimedhas considerable advantages since a very narrow amine hydrochlorideparticle size distribution having a very small mean particle diameter,usually in the nanometer to micron range, can be produced when using astatic mixer, preferably a nozzle, as reactor for the first stage as aresult of the introduction of high mixing energies. However, it is moreadvantageous to achieve high conversions or if possible completeconversion in the phosgenation of amine hydrochloride before solid aminehydrochloride or carbamoyl chloride precipitates in relatively largeamounts, since solids handling is complicated from a process engineeringpoint of view and can lead to caked material and blockages and,secondly, the phosgenation of large and agglomerated amine hydrochlorideparticles as described in the literature is very slow.

The present invention accordingly provides a process for preparingpolyisocyanates by reacting organic amines with phosgene, wherein thereaction is carried out in at least three stages, with the first stagebeing carried out in a mixing apparatus, the second stage in at leastone residence apparatus and the third stage in at least one separationapparatus and the pressure in each successive stage being lower thanthat in the previous stage.

The first stage of the process of the present invention comprisesessentially the reaction of the amine to carbamoyl chloride and aminehydrochloride, the second stage comprises essentially the reaction ofthe amine hydrochloride formed in the first stage to give carbamoylchloride and the third stage comprises essentially the dissociation ofthe carbamoyl chloride into isocyanate and hydrogen chloride.

In the process of the present invention, the reaction between organicamine and phosgene is carried out in three or more stages in an inertsolvent, preferably toluene or chlorobenzene, dichlorobenzene ormixtures thereof, using an excess of phosgene, with the pressure beingreduced from stage to stage. The first phosgenation stage comprises astatic mixer, preferably a nozzle. The pressure upstream of the nozzleis preferably from 3 to 70 bar, in particular from 15 to 45 bar. Thepressure difference over the nozzle is at least 0.5 bar. The temperaturein the first stage is preferably from 80 to 190° C., in particular from90 to 150° C. The second stage comprises one or more residenceapparatuses, preferably one residence apparatus, which is operated at apressure of from 2.5 to 35 bar, preferably from 15 to 35 bar. Downstreamof the nozzle, the reaction mixture is depressurized by means of aregulating valve or some other device suitable for this purpose to thepressure of the residence apparatus of the second stage. However, thenatural pressure drop of the nozzle can also be used for the pressurereduction.

The reactor of the first stage can also be integrated into the reactorof the second stage. In particular, a mixing nozzle can dip into the gasphase or preferably into the liquid phase of the second reactor, i.e.can be located entirely or partly therein. It is also possible for theoutput from the nozzle to be conveyed by means of a pipe, an immersedtube or a plug-in tube into the gas phase or preferably into the liquidphase of the reactor of the second stage.

The temperature in the second stage is from 80 to 190° C., preferablyfrom 90 to 150° C. Possible types of reactor for the second stage aretube reactors, stirred vessels, unstirred residence apparatuses, phaseseparation apparatuses and other apparatuses. The reactor can also beprovided with a pumped circuit which may in turn have a heat exchangerfor setting the reaction temperature. In the case of a stirred vessel,an unstirred residence apparatus or possibly also in the case of a phaseseparation apparatus, the liquid phase is preferably depressurized underlevel control and the gas phase is depressurized under pressure controlinto the reactor of the third stage. However, the gas phase, whichcomprises mainly phosgene, hydrogen chloride and possibly solvent, canalso be passed directly to the work-up, e.g. fractionation intophosgene, hydrogen chloride and solvent or into mixtures thereof. Theresidence reactor of the second stage can, depending on the desiredresidence time and capacity of the plant, have relatively largedimensions and volumes, which can be regarded as disadvantageous fromthe point of view of cost or safety, e.g. phosgene holdup at highpressure. In this case, the reactor of the second stage can be realisedas two or more similar or different reactors and types of reactor, whichcan be connected in parallel or, if appropriate to influence theresidence time spectrum, also in series.

The reactor of the third stage of the process of the present inventionis operated at a pressure of from 2 to 20 bar, preferably from 3.5 to 16bar. Downstream of the residence reactor of the second stage, thereaction mixture is depressurized to the pressure of the third reactorby means of a regulating valve or some other device suitable for thispurpose. A natural pressure drop may also be able to be utilized.

In any case, the pressure in the following stage is, as described above,selected so that it is lower than in the previous stage.

The temperature in the third stage is from 80 to 190° C. The reactorused for the third stage is a column, in particular a reaction column asis described, for example, in WO 99/54289. The temperature at the bottomis from 80 to 190° C. and the temperature at the top is from 50 to 120°C. The column used as reactor of the third stage can also be utilizedfor removing the excess phosgene from the reaction mixture. The reactorof the third stage can, like the reactor of the second stage, bedisadvantageously large. In this case, the reactor of the third stagecan also be realised as two or more similar or different columnsconnected in series. The output from the bottom of the reaction columnis worked up by customary methods to remove any phosgene still presentand to separate off the solvent. In the case of the preparation of TDI,the crude TDI is subsequently subjected to removal of high boilers andpurification by distillation. Phosgene, hydrogen chloride and possiblysolvent can be separated off in a known manner from the vapor leavingthe reaction column and, if appropriate, the residence reactor of thesecond stage and can, if appropriate, be recirculated.

As solvents, preference is given to using chlorinated aromatichydrocarbons such as dichlorobenzene, chlorobenzene, trichlorobenzene ormixtures thereof, aromatic or aliphatic hydrocarbons such as toluene,xylene, benzene, pentane, hexane, heptane, octane, cyclohexane,biphenyl, ketones such as 2-butanone, methyl isobutyl ketone, esterssuch as diethyl isophthalate, ethyl acetate, butyl acetate, nitrilessuch as acetonitrile and also sulfolane.

The particular advantage of the process of the present invention isthat, in contrast to the processes customary in the prior art, the tworeaction steps 1) phosgenation of the amine hydrochloride to formcarbamoyl chloride and 2) decomposition of the carbamoyl chloride intoisocyanate and hydrogen chloride are carried out partly or entirely inseparate stages or reactors and very high chemical yields, very highspace-time yields and at the same time a very low phosgene holdup can beachieved as a result of the independent setting of the optimum pressureand optimum temperature for the respective reaction and the selection ofthe most favorable reactor design in each case. The synthesis can becarried out adiabatically or isothermally. The differing apparatusdesign optimally takes account of the conditions of the two reactions.While the phosgenation of the amine hydrochloride requires highpressures, low pressures are advantageous for the decomposition of thecarbamoyl chloride. Furthermore, the residence times selected for thereactor for the phosgenation of the amine hydrochloride can be shorterthan those for the decomposition of the carbamoyl chloride, whichconsiderably reduces the overall phosgene holdup. Furthermore, removal,in particular by stripping, of the gaseous hydrogen chloride formed isadvantageous for the decomposition of the carbamoyl chloride because thecarbamoyl chloride/isocyanate equilibrium is in this way advantageouslyshifted to the side of the desired isocyanate. This can be taken intoaccount by the choice of a (reaction) column as optimum reactor design.The excess phosgene can also be removed at the same time. This is notabsolutely necessary in this stage, and can also be carried out in afurther stage. In contrast, the removal of hydrogen chloride in theresidence reactor of the second stage would be very disadvantageous,since the phosgene necessary for the phosgenation of the aminehydrochloride would in such a case also be removed together with thehydrogen chloride. Vaporizing phosgene also leads to cooling of thereaction mixture, which could lead to massive precipitation of solidcarbamoyl chloride and amine hydrochloride.

The rapid reaction between amine and phosgene to form carbamoyl chlorideand hydrogen chloride and also amine hydrochloride requires highpressures in both the first and second stages to achieve high phosgeneconcentrations in the liquid phase and thus high excesses of phosgene ifgood chemical yields are to be obtained as a result of low by-productformation. In addition, good mixing is necessary, i.e. a static mixer,preferably a nozzle should be used as apparatus. High admissionpressures upstream of the nozzle allow high pressure drops over thenozzle and thus the introduction of high mixing energies.

Dissolved amine hydrochloride and very small amine hydrochlorideparticles surprisingly react very quickly with phosgene in the residencereactor of the second stage and therefore do not require a longresidence time. High phosgene concentrations are advantageous here.Relatively high pressures do not have an adverse effect on thephosgenation of the amine hydrochloride, so that high phosgeneconcentrations in the liquid phase can advantageously be set by means ofrelatively high pressures. Suitable apparatuses are, in particular, tubereactors, stirred vessels, unstirred residence apparatuses, phaseseparation apparatuses or other residence apparatuses. Furthermore,heatable embodiments of these apparatuses are advantageous in order tocompensate, if appropriate, temperature drops caused by the endothermicdecomposition of the carbamoyl chloride. The reaction of the thirdstage, viz. the dissociation of the carbamoyl chloride into isocyanateand hydrogen chloride, is a pressure-dependent equilibrium reaction. Itis advantageouslly shifted to the side of the desired isocyanate by lowpressures. Since this reaction does not require any phosgene, the lowphosgene concentrations in the liquid phase characteristic of lowpressures do not interfere and in fact lead to a low phosgene holdup inthe reactor of the third stage, which is advantageous from a safetypoint of view. In this way, the overall phosgene holdup in the process,and possibly also in the individual apparatuses, can be significantlyreduced compared to a cascade of stirred vessels or a reaction tower.High pressures are, in contrast, very unfavorable for the dissociationof carbamoyl chloride and require long residence times, hightemperatures and energy consumptions, since at high pressures theequilibrium lies far on the side of the carbamoyl chloride. Longresidence times in turn result in a high phosgene holdup. A (reaction)column as described, for example, in WO 99/54289 (DE 19817691) isparticularly useful as apparatus for the third stage. In addition thestripping effect shifts the carbamoyl chloride/isocyanate equilibriumadvantageously to the side of the desired isocyanate.

The phosgenation of the amine hydrochloride does not have to becompleted in the second stage, and it is likewise possible for thedecomposition of the carbamoyl chloride to start in the second stage.However, preference is given to a design of the reactor of the secondstage in terms of residence time and other process engineeringparameters which is such that the phosgenation of the aminehydrochloride is virtually complete and the decomposition of thecarbamoyl chloride has not yet progressed to any large extent.

If the phosgenation of the amine hydrochloride and the decomposition ofthe carbamoyl chloride are, as described in the prior art, carried outin one stage or in one reactor, the high pressure required for thephosgenation of the amine hydrochloride results in a low conversion ofthe carbamoyl chloride into the isocyanate and thus long residencetimes. A high phosgene concentration and long residence times (largereaction volumes) in turn result in a very high phosgene holdup. This ispresent at high pressures and temperatures which are of concern from asafety point of view. Physical separation of the two reactions, viz. thephosgenation of the amine hydrochloride in the second phosgenation stageat a high pressure and the decomposition of the carbamoyl chloride inthe third phosgenation stage at a low pressure, enables high chemicalyields, high space-time yields and especially a low phosgene holdup inthe overall process and possibly also in the individual apparatuses tobe achieved.

Physical separation of the first and second stages is not absolutelynecessary, since a high pressure increases the phosgene concentration inthe liquid phase, which favors both the first reaction between amine andphosgene and the second reaction between amine hydrochloride andphosgene.

Furthermore, the process can be carried out at elevated temperature andif desired also isothermally in all stages. This results, in particular,in high space-time yields and thus low phosgene holdups and smallerapparatuses together with simultaneously higher chemical yields,especially compared to the classical cold/hot phosgenation. In addition,avoiding cooling of the reaction mixture in the first stage andsubsequent reheating in the second stage and the following stages savesa considerable quantity of energy. Avoiding the precipitation of aminehydrochloride as solid enables long residence times, which can sometimesbe realised only by means of a circulation mode of operation (loopreactors), to be avoided. Although the circulation mode of operationlikewise involves a lower phosgene holdup than, for example, a cascadeof stirred vessels, it suffers from increased formation of by-products,in particular ureas. To avoid urea formation, the amine or isocyanateconcentration has to be kept low, which leads to very low space-timeyields.

The temperatures and pressures employed are to some extent dependent onthe amine used. Likewise, the phosgene excesses and residence times tobe employed in the individual apparatuses are dependent on the amineused. In the case of diphenylmethane diisocyanates (MDI) and/orpolyphenylene-polymethylene polyisocyanates (PMDI) or mixtures of thesetwo, the excess of phosgene should be at least 100% of thestoichiometric amount, while in the case of tolylene diisocyanate (TDI),it should at least 300% of the stoichiometric amount and in the case ofhexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) itshould likewise be at least 300% of the stoichiometric amount. Theresidence time in the first stage (static mixer) is naturally very shortand is defined by the apparatus design. The mean residence time in theresidence apparatus of the second stage can be from one second to 30minutes. Preference is given to mean residence times of from 30 secondsto 10 minutes, particularly preferably from 2 to 7 minutes. The meanresidence time in the apparatus of the third stage (reaction column) isdependent on the number of theoretical plates, the height of the weir,the volume of liquid phase, the throughput through the column and otherprocess engineering parameters. It is preferably not more than 1 hour.

The amine concentration in the inert solvent is likewise dependent onthe amine used and on the solvent. In the case of diphenylmethanediamine(MDA) and/or polyphenylene-polymethylenepolyamines (PMDA) or mixtures ofthese two, it can be 5-50% by weight, preferably 25-40% by weight; inthe case of toluenediamine (TDA) it can be 5-50% by weight, preferably15-30% by weight; in the case of hexamethylenediamine (HDA) it can be5-50% by weight, preferably 15-30% by weight; and in the case ofisophoronediamine (IPDA) it can likewise be 5-50% by weight, preferably15-30% by weight. The phosgene concentration in the inert solvent can be0-70% by weight, preferably 10-50% by weight. Preference is given tousing the same solvent as for the amine. It is also possible to omit asolvent entirely.

The invention is illustrated by the following examples.

EXAMPLES

1a) Tube Reactor as Residence Apparatus (I)

A solution of 0.73 kg/h of toluenediamine (TDA) in 3.3 kg/h ofchlorobenzene was reacted at 110° C. with 6.2 kg/h of phosgene in amixing nozzle. The pressure drop over the nozzle was 6 bar. The reactionmixture was depressurized directly into a tube reactor heated by meansof a double wall as residence apparatus having a residence time of 2minutes at 110° C. and a pressure of 15 bar abs. The output from thetube reactor was depressurized via a regulating valve into a bubble captray column (reaction column). The temperature at the bottom of thereaction column was 150° C. and the pressure at the top was 3.5 bar abs.At the top of the column, a mixture (5.8 kg/h) of phosgene (4.8 kg/h,82% by weight), hydrogen chloride (0.85 kg/h, 15% by weight) andchlorobenzene (0.19 kg/h, 3% by weight) and small amounts of various lowboilers (CCl₄, CHCl₃, N₂, CO, CO₂) was taken off, partially condensedand passed to hydrogen chloride/phosgene separation by a known method.Part of the condensate was returned to the column as runback. Thetemperature at the top was 71° C. At the bottom of the column, a mixture(4.4 kg/h) of tolylene diisocyanate (1.0 kg/h, 23% by weight),chlorobenzene (3.1 kg/h, 70% by weight), phosgene (0.27 kg/h, 6% byweight), hydrogen chloride (0.02 kg/h, 0.5% by weight) and small amountsof high boilers (0.04 kg/h, 1% by weight) was taken off. Ashell-and-tube apparatus having 13 tubes was used as bottom circulationvaporizer.

1b) Tube Reactor as Residence Apparatus (II)

A solution of 0.73 kg/h of toluenediamine (TDA) in 3.2 kg/h ofchlorobenzene was reacted with 6.2 kg/h of phosgene in a mixing nozzle.The pressure drop over the nozzle was 8 bar. The reaction mixture wasdepressurized directly into a tube reactor as residence apparatus havinga residence time of 10 seconds at 120° C. and a pressure of about 15 barabs. The output from the tube reactor flowed directly into a bubble captray column. The temperature at the bottom of the reaction column was150° C. and the pressure at the top was 15 bar abs. At the top of thecolumn, a mixture (3.0 kg/h) of phosgene (2.1 kg/h, 71% by weight) andhydrogen chloride (0.85 kg/h, 29% by weight) and small amounts ofchlorobenzene and various low boilers (CCl₄, CHCl₃, N₂, CO, CO₂) wastaken off, partially condensed and passed to hydrogen chloride/phosgeneseparation by a known method. Part of the condensate was returned to thecolumn as runback. At the bottom of the column, a mixture (kg/h) oftolylene diisocyanate (1.0 kg/h, 14% by weight), chlorobenzene (3.2kg/h, 45% by weight), phosgene (2.9 kg/h, 41% by weight) and smallamounts of high boilers (0.05 kg/h, 1% by weight) was taken off. Ashell-and-tube apparatus having 13 tubes was used as bottom circulationvaporizer.

2) Stirred Vessel as Residence Apparatus

A solution of 0.73 kg/h of toluenediamine (TDA) in 3.3 kg/h ofchlorobenzene was reacted at 140° C. with 6.2 kg/h of phosgene in amixing nozzle. The pressure drop over the nozzle was 4 bar. The nozzlewas integrated into a tube reactor as residence reactor into which thereaction mixture was depressurized. The residence reactor was a stirredvessel which was brought to a temperature of 140° C. via the doublewall. The stirrer speed was 1000 revolutions per minute. As analternative, the nozzle was placed outside the residence reactor and theoutput from the nozzle was fed directly into the liquid phase via aplug-in tube. Depressurization into the gas phase led to somewhat loweryields of tolylene diisocyanate (TDI). The liquid phase is dischargedunder level control and the gas phase is discharged under pressurecontrol from the residence reactor into a bubble cap tray column(reaction column). The stirred vessel was operated in the pressure rangefrom 2.5 to 35 bar abs. The mean residence time of the liquid phase(regulated via the level) was up to 30 minutes. The stirred vessel wasalso operated with an external loop provided with a heat exchanger(pumped circuit with gear pump). The temperature at the bottom of thereaction column is 110° C. and the pressure at the top is 3.5 bar abs.At the top of the column, a mixture (5.0 kg/h) of phosgene (4.0 kg/h,20% by weight), hydrogen chloride (0.85 kg/h, 17% by weight) andchlorobenzene (0.16 kg/h, 3% by weight) and small amounts of various lowboilers (CCl₄, CHCl₃, N₂, CO, CO₂) is taken off, partially condensed andpassed to hydrogen chloride/phosgene separation by a known method. Partof the condensate was returned to the column as runback. The temperatureat the top was 70° C. At the bottom of the column, a mixture (5.2 kg/h)of tolylene diisocyanate (1.0 kg/h, 19% by weight), chlorobenzene (3.1kg/h, 59% by weight), phosgene (1.15 kg/h, 22% by weight), hydrogenchloride (0.02 kg/h, 0.3% by weight) and small amounts of high boilers(0.02 kg/h, 0.4% by weight) was taken off. A shell-and-tube apparatushaving 13 tubes was used as bottom circulation vaporizer.

3) Phase Separation Apparatus

A solution of 0.74 kg/h of diaminodiphenylmethane (MDA) in 1.6 kg/h ofchlorobenzene was reacted with a solution of 1.9 kg/h of phosgene in 2.1kg/h of chlorobenzene in a mixing nozzle. The pressure drop over thenozzle was 5 bar. The nozzle was depressurized via a plug-in tube intothe liquid phase of a phase separator. The pressure downstream of thenozzle was 12 bar and the temperaturee was 115° C. The liquid phase andthe gas phase were conveyed separately into a bubble cap tray column(reaction column). The mean residence time of the liquid phase was about3 minutes. The temperature at the bottom of the reaction column was 115°C. and the pressure at the top was 5 bar abs. At the top of the column,a mixture (1.5 kg/h) of phosgene (0.73 kg/h, 50% by weight), hydrogenchloride (0.50 kg/h, 33% by weight) and chlorobenzene (0.24 kg/h, 16% byweight) and small amounts of various low boilers (CCl₄, CHCl₃, N₂, CO,CO₂) was taken off, partially condensed and passsed to hydrogenchloride/phosgene separation by a known method. Part of the condensatewas returned to the column as runback. The temperature at the top was110° C. At the bottom of the column, a mixture (4.9 kg/h) ofmethylenedi(phenyl isocyanate) (MDI, 0.93 kg/h, 19% by weight),chlorobenzene (3.5 kg/h, 71% by weight), phosgene (0.43 kg/h, 9% byweight) and hydrogen chloride (0.05 kg/h, 1.0% by weight) was taken off.A shell-and-tube apparatus having 13 tubes was used as bottomcirculation vaporizer.

1. A process for preparing at least one polyisocyanate comprisingreacting organic amines with phosgene in an inert solvent, wherein thereaction is carried out in at least three stages, with the first stagebeing carried out in a mixing apparatus, the second stage in at leastone residence apparatus and the third stage in at least one reactioncolumn and the pressure in each successive stage being lower than thatin the previous stage.
 2. The process of claim 1, wherein the at leastone polyisocyanate is diphenylmethane diisocyanate (MDI),polyphenylene-polymethylene polyisocyanate (PMDI), tolylene diisocyanate(TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI),or a mixture of diphenylmethane diisocyanate (MDI) andpolyphenylene-polymethylene polyisocyanate (PMDI).
 3. The process ofclaim 1, wherein a nozzle is used as the mixing apparatus for the firststage.
 4. The process of claim 1, wherein a tube reactor, a stirredvessel, an unstirred residence apparatus or a phase separation apparatusfor gas and liquid phases is used as the at least one residenceapparatus for the second stage.
 5. The process of claim 1, wherein theresidence time in the residence apparatus of the second stage is from 1second to 30 minutes.
 6. The process of claim 1, wherein the at leastone residence apparatus of the second stage comprises at least tworeactors of the same or different types which are connected in parallel,in series, or in a combination thereof.
 7. The process as of claim 1,wherein the phosgene is separated off in the at least one reactioncolumn of the third stage.
 8. The process of claim 1, wherein thepressure is reduced from the pressure of the mixing apparatus of thefirst stage to the pressure of the residence apparatus of the secondstage by a regulating device.
 9. The process of claim 1, wherein thepressure is reduced from the pressure of the at least one residenceapparatus of the second stage to the pressure of the at least onereaction column of the third stage by a regulating device.
 10. Theprocess of claim 1, wherein the mixing apparatus of the first stage isintegrated into the at least one residence apparatus of the secondstage.
 11. The process of claim 1, wherein the pressure upstream of themixing apparatus is 3-70 bar.
 12. The process as of claim 1, wherein thetemperature in the first, second and third stages is in each case80-190° C.
 13. The process of claim 1, wherein an aromatic hydrocarbon,a chlorinated aromatic hydrocarbon, or a mixture thereof is used as theinert solvent.
 14. The process of claim 1, wherein the residence time inthe at least one residence apparatus of the second stage is from 30seconds to 10 minutes.
 15. The process of claim 1, wherein the residencetime in the at least one residence residence apparatus of the secondstage is from 2 to 7 minutes.
 16. The process of claim 8, wherein theregulating device is a regulating valve.
 17. The process of claim 9,wherein the regulating device is a regulating valve.
 18. The process ofclaim 1, wherein the pressure upstream of the mixing apparatus is 15-45bar.
 19. The process of claim 1, wherein the pressure in the reactor ofthe second stage is 2.5-35 bar.
 20. The process of claim 1, wherein thetemperature in the first, second and third stages is in each case90-150° C.